Significance Implantation of the blastocyst in the maternal endometrium follows a specific sequence in different species of mammals. Nonhuman primates show similarities between species and with the human with regard to syncytial trophoblast formation, formation of the trophoblastic plate, and lacunar and villus formation. We have determined that the cytotrophoblast involved in formation of the trophoblastic shell demonstrates surface features which appear to enable this trophoblast to invade maternal arteries as well as endometrial stroma. Objective The objective of this project is to understand more completely the processes involved in implantation of the blastocyst in the endometrium in non-human primates. Studies focus on cellular interactions of the trophoblast and endometrium, and on differentiation of the earlypostimplantation embryo and trophoblast. Results Macaque implantation sites were examined by electron microscopy and immuno-cytochemistry, focussing on modification of the wall of maternal arteries by endovascular cytotrophoblast. Immunostaining of cell adhesion molecules combined with localization of constituents that indicate cell type showed a sequence of changes in the vessel wall as cytotrophoblast migrated and invaded. Electron microscopic examination of the vessels showed that intramural cytotrophoblast secretes a matrix which excludes maternal elements, resulting in an intramural pad of cytotrophoblast. The studies also described the way in which the arteries, especially the muscle layers of the vessels, become remodeled. Future Directions Although cytotrophoblast modifies maternal arteries, the first invasion of the uterine epithelium is by syncytial trophoblast. It is not yet known what cell surface factors facilitate invasion of this epithelium and of maternal vessels. We will attempt to obtain some pre-lacunar implantation stages in order to assess the invasive characteristics of trophoblast types in the first days of implantation. KEYWORDS placenta, trophoblast, endometrium, immunocytochemistry